How Does a Proportioning Valve Work? And How Do You...

When setting up a street car for performance driving or converting a classic to disc brakes, it is essential to include an adjustable proportioning valve in your brake system. This valve is typically plumbed into the rear brake line or built into the combination valve below the master cylinder. There is some confusion, though, about what these valves do, how they do it, and how to properly adjust them.

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With a typical tandem master cylinder, the amount of brake line pressure delivered to each outlet will always be the same without a proportioning valve. However, since the rear wheels have less traction and contribute less to braking, you need a way to adjust that pressure. Race cars with separate front and rear master cylinders use different bore sizes in the calipers and master cylinders so that the rear receives less pressure at all times. Even these cars sometimes employ a proportioning valve for finer adjustments.

Many people misinterpret the function of a proportioning valve, thinking it works like a simple pressure regulator. In reality, it decreases the pressure going through it by a set amount once the initial pressure reaches a certain point. This adjustment does not increase the pressure in the other brake circuit; it merely changes the balance between the front and rear braking forces.

Adjusting the valve does not directly change the pressure that reaches the brakes; it modifies the pressure level at which the valve activates, often referred to as the "knee point" on a brake pressure graph. Counterintuitively, turning the knob or lever all the way out results in the greatest reduction in maximum pressure. Turning it inward allows more pressure to reach the rear brakes before the proportioning mechanism engages.

In Normal Driving

Under normal street driving conditions, you won't often use the brakes hard enough to risk locking them up. During typical driving, the proportioning valve does little to nothing. At lower brake pressures, the rear brakes take on a larger share of the braking because less weight transfer to the front means less chance of rear wheel lockup.

This setup also saves wear on the front brake pads by letting the rear brakes handle more of the braking under light pressure. In a hard stop, you want the front wheels to lock slightly before the rear to prevent a spin.

A front-wheel brake bias, whether achieved with caliper piston sizes, a proportioning valve, or different master cylinders, serves to reduce line pressure going to the rear brakes to prevent locking up. This is crucial because braking shifts most of the vehicle's weight to the front wheels, reducing the rear wheels' traction and their ability to handle pressure without locking up.

The proportioning valve allows you to adjust the rear brake pressure for different tires, weight distribution, and brake types.

How it Works

The inner workings of an adjustable proportioning valve are deceptively simple. Inside, a piston with variable surface area and a spring controlled by a knob or lever dictate the proportioning rate.

In normal driving, the valve does nothing, allowing 100% of brake line pressure to pass through. When the force on the larger piston area (outlet to rear brakes) exceeds the spring preload, the piston shifts, cutting off the inlet from the outlet. Once unseated, pressure from the master cylinder moves the larger piston, transmitting pressure to the rear brakes. This cycle happens rapidly, too quickly to notice but confirmed by engineers in lab settings.

The different piston ends create a hydraulic disadvantage. For example, 1000 psi on the smaller piston results in just 430 psi on the larger piston side. Adjusting the valve changes the spring preload, altering the point where pressure reduction begins and modifying the 'knee point' on a pressure graph.

Wilwood Proportioning Valves

Wilwood manufactures three styles of adjustable proportioning valves, all based on the same internal design with varying strengths. Their adjustable proportioning valve activation range spans from approximately 150 psi to 1200 psi.

The knob style valve allows infinite adjustments, while the lever style has six different settings for quick adjustments during racing. The pressure to the rear brakes remains proportional to the master cylinder pressure. Adjusting the valve changes the point of proportioning without altering the relationship itself.

Set midway on the knob, the valve triggers at 675 psi. From 1-675 psi, 100% of that pressure goes to the rear calipers. Beyond that, only 43% of additional pressure is transmitted. For instance, if the master cylinder outputs 1000 psi, the rear brake lines would receive 815 psi.

Why You Need One

Even though a brake lockup event is rare, it can be catastrophic. Keeping your rear brakes from locking up is crucial, especially in performance or conversion vehicles with mismatched brake setups.

Most disc brake conversions include discs in the front and drums in the rear, upsetting the factory brake balance. Adding an adjustable valve allows you to fine-tune rear braking pressure to suit the new system.

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Installing four-wheel discs on a former drum brake car without an adjustable valve will likely result in poor rear braking because drum brakes lock up at much lower pressures. Factory non-adjustable valves work within narrow confines and may also include a residual pressure valve that should be removed.

Early disc/drum cars used low-pressure proportioning valves to prevent rear lockups. Modern vehicles with computer-controlled stability systems dynamically adjust brake bias, but older systems rely on fixed settings, making an adjustable valve essential.

Proper Initial Adjustment

For safety, start with the proportioning valve at its maximum reduction setting and work from there. With an assistant and an empty lot, repeatedly stop from 30 mph to lock up the wheels and find the optimal settings.

• Ensure tire pressure is correct.
• Half-tank of gas or less, no passengers, minimal trunk load.
• Accelerate to 30 mph and stop hard enough to lock the wheels.
• Have an assistant observe which wheels lock first.
• If only the front locks, adjust the knob inward to allow more pressure to the rear brakes.
• Repeat until both front and rear lock simultaneously in a hard stop.
• Adjust the knob out one turn and test again.
• Continue adjusting 1/2 turn at a time for optimal braking with no rear lock.
• Test at 50 mph for more dramatic weight transfer and make further adjustments as needed.

When bleeding brakes, turn the valve all the way in to make the process easier. Mark the number of rotations to fully close, then adjust back out the same amount after bleeding.

Proportional Solenoid Valve Controller - How They Work

Proportional Solenoid Valve Controller - How They Work

Proportional solenoid valves regulate fluid flow rate by varying the plunger position, impacting pressure, level, and temperature control. These valves are typically normally closed but can also be normally open. This article will discuss normally closed proportional solenoid valves.

With power, the plunger moves up against the spring to open the valve. Without power, the spring closes the valve. Unlike standard solenoids, proportional solenoid valves allow plunger position control across a range of stroke positions by varying power to the solenoid coil.

The Burkert 8611 and 8605 controllers use pulse-width modulation (PWM) to control these valves, offering configurable control functions to ensure accurate process control.

Pulse Width Modulation (PWM)

PWM is a digital signal that switches ON and OFF to control valve position. The switching frequency and duty cycle influence the effective current delivered to the solenoid.

Although PWM signals are square, the current delivered creates a saw tooth curve due to coil inductance and duty cycle, causing the plunger to oscillate in a balanced state (dither). This reduces static friction and decreases hysteresis.

Valve performance is most sensitive in a specific switching frequency range with an optimal duty cycle. The working point is where the valve is most responsive, with known switching frequency limits ensuring correct operating parameters.

Burkert's controllers memorize frequency limits for their valves, ensuring correct configuration automatically. When using other brand valves, correct operating parameters must be inputted.

PI (Proportional and Integral) Control Theory

A PI control loop adjusts a control variable (like PWM frequency) based on feedback. The system error determines proportional and integral terms, dynamically adjusting the control variable to reduce error. The 8611 model uses closed-loop control, while the 8605 uses open-loop control.

The proportional term is the product of a tuning factor and the difference between setpoint and process variable. The integral term accounts for error over time, stabilizing system error by regulating the control variable.

PI control loops provide reduced hysteresis, automated error correction, and increased process stability. Proper tuning is crucial for optimal performance.

Contact us to discuss your requirements with a hydraulic check valves manufacturer. Our experienced sales team can help you identify the options that best suit your needs.

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